Optimize write checkpoint lookups when users have none (#355)

* Minor cleanup.

* Only query write checkpoint once initially.

* Changeset.

Author: rkistner

.changeset/wet-grapes-study.md

@@ -0,0 +1,7 @@
+---
+'@powersync/service-module-mongodb-storage': patch
+'@powersync/service-core': patch
+'@powersync/service-image': patch
+---
+
+Avoid frequent write checkpoint lookups when the user does not have one.

modules/module-mongodb-storage/src/storage/implementation/MongoSyncBucketStorage.ts

@@ -714,22 +714,28 @@ export class MongoSyncBucketStorage
     let lastCheckpoint: ReplicationCheckpoint | null = null;
 
     const iter = this.sharedIter[Symbol.asyncIterator](options.signal);
+
     let writeCheckpoint: bigint | null = null;
+    // true if we queried the initial write checkpoint, even if it doesn't exist
+    let queriedInitialWriteCheckpoint = false;
 
     for await (const nextCheckpoint of iter) {
       // lsn changes are not important by itself.
       // What is important is:
       // 1. checkpoint (op_id) changes.
       // 2. write checkpoint changes for the specific user
 
-      if (nextCheckpoint.lsn != null) {
-        writeCheckpoint ??= await this.writeCheckpointAPI.lastWriteCheckpoint({
+      if (nextCheckpoint.lsn != null && !queriedInitialWriteCheckpoint) {
+        // Lookup the first write checkpoint for the user when we can.
+        // There will not actually be one in all cases.
+        writeCheckpoint = await this.writeCheckpointAPI.lastWriteCheckpoint({
           sync_rules_id: this.group_id,
           user_id: options.user_id,
           heads: {
             '1': nextCheckpoint.lsn
           }
         });
+        queriedInitialWriteCheckpoint = true;
       }
 
       if (
@@ -739,12 +745,13 @@ export class MongoSyncBucketStorage
       ) {
         // No change - wait for next one
         // In some cases, many LSNs may be produced in a short time.
-        // Add a delay to throttle the write checkpoint lookup a bit.
+        // Add a delay to throttle the loop a bit.
         await timers.setTimeout(20 + 10 * Math.random());
         continue;
       }
 
       if (lastCheckpoint == null) {
+        // First message for this stream - "INVALIDATE_ALL" means it will lookup all data
         yield {
           base: nextCheckpoint,
           writeCheckpoint,
@@ -758,7 +765,9 @@ export class MongoSyncBucketStorage
 
         let updatedWriteCheckpoint = updates.updatedWriteCheckpoints.get(options.user_id) ?? null;
         if (updates.invalidateWriteCheckpoints) {
-          updatedWriteCheckpoint ??= await this.writeCheckpointAPI.lastWriteCheckpoint({
+          // Invalidated means there were too many updates to track the individual ones,
+          // so we switch to "polling" (querying directly in each stream).
+          updatedWriteCheckpoint = await this.writeCheckpointAPI.lastWriteCheckpoint({
             sync_rules_id: this.group_id,
             user_id: options.user_id,
             heads: {
@@ -768,6 +777,9 @@ export class MongoSyncBucketStorage
         }
         if (updatedWriteCheckpoint != null && (writeCheckpoint == null || updatedWriteCheckpoint > writeCheckpoint)) {
           writeCheckpoint = updatedWriteCheckpoint;
+          // If it happened that we haven't queried a write checkpoint at this point,
+          // then we don't need to anymore, since we got an updated one.
+          queriedInitialWriteCheckpoint = true;
         }
 
         yield {

modules/module-mongodb-storage/src/storage/implementation/MongoWriteCheckpointAPI.ts

@@ -12,12 +12,10 @@ export type MongoCheckpointAPIOptions = {
 export class MongoWriteCheckpointAPI implements storage.WriteCheckpointAPI {
   readonly db: PowerSyncMongo;
   private _mode: storage.WriteCheckpointMode;
-  private sync_rules_id: number;
 
   constructor(options: MongoCheckpointAPIOptions) {
     this.db = options.db;
     this._mode = options.mode;
-    this.sync_rules_id = options.sync_rules_id;
   }
 
   get writeCheckpointMode() {